1. Field of the Invention
The present invention relates to an improvement in the structure of a miniaturized oxygen electrode which can be mass-produced at a low cost.
A miniaturized oxygen electrode can be advantageously used for the measurement of oxygen concentration in various fields. For example, the miniaturized oxygen electrode is used for the measurement of biological oxygen demand (BOD) in water from the viewpoint of water control. In the fermentation industry, the regulation of the dissolved oxygen concentration in the fermentation tank is necessary for fermentation with a high efficiency. A miniaturized oxygen electrode is used as a measuring instrument for this purpose. Further, a miniaturized oxygen electrode can be combined with an enzyme to form a biosensor which can be used for the measurement of the concentration of sugar or alcohols. For example, glucose is reacted with dissolved oxygen in the presence of a catalyst of glucose oxidase, an enzyme, causing the glucose to be oxidized to gluconolactone. This reduces the amount of dissolved oxygen diffusing into an oxygen electrode. By taking advantage of this phenomenon, the glucose concentration can be measured based on the consumption of dissolved oxygen.
Thus, the miniaturized oxygen electrode is utilized in various fields, including environmental instrumentation, the fermentation industry, and clinical care. In particular, in medical care, the miniaturized oxygen electrode as the glucose sensor for patients suffering from diabetes is of high utility value because it is small, disposable and low cost.
2. Description of the Related Art
Glass or vinyl chloride oxygen electrodes of Clark type have been used as the oxygen electrode. However, the electrodes could neither be miniaturized nor mass-produced.
For this reason, the present inventors have proposed a new type of miniaturized oxygen electrode that can be produced through the utilization of lithography and anisotropic etching (see Japanese Unexamined Patent Publication (Kokai) No. 63-238548: Japanese Examined Patent Publication (Kokoku) No. 6-1254). This miniaturized oxygen electrode comprises a structure produced by forming a hole on a silicon substrate by anisotropic etching, forming two electrodes through an insulating layer on the bottom of the hole, filling the hole with an electrolyte-containing material and finally covering the upper surface of the hole with a gas-permeable membrane. Further, the present inventors have developed a technique where an electrolyte-containing material and a gas-permeable membrane are provided only in a requisite place by taking advantage of screen printing (Japanese Unexamined Patent Publication (Kokai) No. 5-87766). This miniaturized oxygen electrode has a small size, gives rise to no scattering of properties and can be collectivelymass-produced, which renders the production cost low.
FIGS. 1A and 1B are respectively a plan view of the conventional miniaturized oxygen electrode of the type described above and a cross-sectional view taken on line 1B--1B of FIG. A1 In the drawing, numeral 1 designates a miniaturized oxygen electrode. The miniaturized oxygen electrode 1 comprises a rectangular silicon substrate 11 having on its whole surface an insulating layer of SiO.sub.2 comprising a pair of a cathode pattern 12 and an anode pattern 13 each as a silver layer, the cathode pattern 12 and the anode pattern 13 comprising an active section 12A, 13A, a terminal 12C, 13C for external connection, and a lead wire 12B, 13B for the connection thereof. The active portions 12A, 13A are connected to each other through an electrolyte-containing material 14. The active portion 12A of the cathode pattern 12 serves substantially as a cathode, while the active portion 13A of the anode pattern 13 serves substantially as an anode.
In the region where the cathode 12A overlaps the electrolyte-containing material 14, these elements are in contact with each other through an opening 15 provided in a hydrophobic layer (not shown), interposed between the elements, such as a photoresist, and an oxygen-sensitive section (a measuring section) 12S is defined by the opening 15.
In the region where the anode 13A overlaps with the electrolyte-containing material 14, these elements are in contact with each other through an opening 17 provided in a water-impermeable membrane 16, such as a polyimide, interposed between both the elements.
The electrolyte-containing material 14 is covered with an oxygen-permeable membrane 18 (FIG. 1B). The oxygen-permeable membrane 18 covers the whole substrate region except for the substrate region at the portion which includes terminals (pads) 12C, 13C (i.e., in FIG. 1A, the whole region above the terminals 12C, 13C is covered.)
When the miniaturized oxygen electrode shown in FIGS. 1A and 1B is used, a certain voltage, which is negative with respect to the anode, is applied to the cathode. The immersion of the sensitive section in this state in a buffer causes dissolved oxygen to pass through the oxygen- or gas-permeable membrane and reach the working electrode (cathode), where the dissolved oxygen is reduced. A current generated by the reduction reaction can be measured to determine the dissolved oxygen concentration using the current value as a measure.
Further, in order to render the miniaturized oxygen electrode more suitable for actual production in a plant and, at the same time, to further improve the performance, the present inventors have developed, as a miniaturized oxygen electrode of the above type, a miniaturized oxygen electrode using a combination of anisotropic etching with anodic bonding (Japanese Unexamined Patent Publication (Kokai) No. 4-125462).
In all the above miniaturized oxygen electrode production techniques, lithography is carried out using as materials a silicon substrate and a glass substrate. Therefore, even though optimal conditions in respect of equipment and the like are prepared and the demand is about several ten million chips per year, it is impossible to reduce the cost to a low value compatible to that of test paper, and not more than 100 yen per chip. For this reason, in order to further reduce the cost, the present inventors have developed a miniaturized oxygen electrode wherein a plastic substrate was used, an electrolyte was impregnated into paper and a gas-permeable membrane was adhered to a sensitive section (Japanese Unexamined Patent Publication (Kokai) No. 6-34596).
The above conventional miniaturized oxygen electrode is much smaller than the old Clark oxygen electrode. In recent years, however, in the field of medical care, for example, there is an increasing demand for the insertion of an oxygen electrode into a blood vessel of a patient to directly measure various components present in the blood. In such applications, the size of the miniaturized oxygen electrode should be reduced to, for example, not more than 1 mm in width. In the case of the conventional miniaturized oxygen electrode shown in FIGS. 1A and 1B, the size could not be reduced to less than about 2 mm in width.